Racket string



y 1938. F. D. CHITTENDEN 1 m. 8, 64

RACKET STRING Filed Aug. 25, 1937 I INVENTORS 660277; 0. CV/fff/Vflf/V ATTORNEYS I Patented May 24, 1938 UNITED STATES PATENT OFFICE RAOKET STRING poration of Delaware 7 Application August 25, 1937, Serial No. 160,788

4 Claims.

This invention relates to a composite racket string which is approximately as strong as a high grade gut string of the same size and is sumciently resilient and durable to remain tight and alive when used in a racket.

Tennis rackets have heretofore been commonly strung with carefully prepared gut strings, and while high grade gut when strung tight forms an excellent racket, such strings are expensive and are injuriously aiiected by the moisture of the atmosphere. Silk racket strings have also been employed heretofore and while silk strings are cheaper than gut and are less sensitive to moisture, they lack the resiliency and aliveness of gut strings.

The present invention contemplates a strong, durable and resilient string that is well adapted for use in rackets as a substitute for gut and which compares favorably to gut in strength, durability and resiliency, while it is less sensitive to moisture than gut and is more resilient or alive than the silk strings proposed heretofore.

Ii'he string of the present invention comprises a rubber core having a strong outer cover formed of a number of cords, preferably of silk, tightly Wrapped helically about the core at a relatively small angle to the length of the core. The rubber core forms a yielding or cushion-like central element about which the cords are tightly wound. This forms a racket string wherein the surrounding cords exert a circumferential compression upon the rubber core when the racket string is tensioned and the tendency of this compressed core to expand within the cover and thereby contract the string render the racket string resilient or slightly elastic. In this manner a racket string is produced which while strong and durable possesses the highly desirable property of resiliency or aliveness.

In carrying out the present invention it is important that the cords forming the cover for the rubber core be tightly bonded to the core and to the contacting cords to thereby firmly unite the elements forming the composite racket string. It is also important that this binder be not brittle but sufliciently resilient to accommodate the stretching and contraction of the racket string.

Furthermore, it is desirable that the racket string be provided with a smooth slick-finish outer surface which is tough and durable and which will serve to protect the interior of the string from the moisture of the atmosphere.

The construction of the present invention whereby these desirable features are secured and .having the cord ii wound thereupon.

other advantages are obtained will be more fully understood from the following description when read in connection with the accompanying drawing, wherein- Fig. 1 is a side elevation of a racket string constructed in accordance with the present invention.

Fig. 2 is an enlarged sectional view taken on line 2-4 of Fig. 1.

Fig. 3 is a side elevation of the racket string 01' Fig. 1 in the course of construction; and

Fig. 4 is a plan view of a tennis racket stru with the string of Fig. l.

The resilient string of the present invention may be employed in tennis, squash and badminton rackets, and for other purposes, and as shown in Fig. 3 comprises a rubber core l about which is helically wound in one direction a number of cords ii, eight of these cords being shown in the construction illustrated. Since a high degree of strength is a primary requisite of the cords i I, it is contemplated that these cords will be formed of a number of finer strands I2 of silk, although other textile filaments may be used, and the number of cords it provided may, of course, be

varied.

The cords ii are wrapped tightly about the core it as a cover while the core is in a stretched condition, and the construction shown in Fig. 3 was produced on an ordinary braiding machine of a sixteen carrier type, only eight of these carriers being employed. That is only the carriers that travel in one direction were used, and each of these carriers was provided with a spool Other types of machine such as, for example, a cord or rope making machine may, however, be employed to make the construction of Fig. 3 provided the mechanism is supplied with means for maintaining the desired tension upon the rubber core during the core covering operation.

It is important that the cords l I be wrapped tightly about the core Ill so that as the racket string thus produced is stretched the tension upon these cords will cause them to circumferentially compress the rubber core It in order that the expanding pressure of this circumferentially compressed core within its cover will tend to expand its cover and thereby contract the cover lengthwise. Thereiore, while the cords H are being wrapped about the core I0 these cords should be maintained under tension, as should also the racket string while it is being formed in the machine to thereby maintain these cords snugly in place about the core. After this racket string I4 leaves the forming machine, it should be well stretched so that it will not elongate unduly when being strung in a racket.

It is important as above stated that the cords II forming the cover be firmly united to the core I0 and to the adjacent cords of the racket string illustrated. To this end in making the construction shown, the core I0 is passed through a supply of adhesive to deposit a coating of adhesive I3 thereupon before the covering cords II are wrapped about this core. When the cords I I are wrapped tightly about the adhesive covered core In this adhesive I3 will squeeze out between the contacting cords to provide a binder therebetween to thereby secure these cords tightly to each other and to the core. Any excess adhesive which accumulates on the outer surface of the racket string should be wiped off, but the adhesive I3 that reaches the outer surface of the cover and lies within the crevices between the helically wrapped cords will serve to fill up these crevices and give the cover the relatively smooth outer surface indicated by I5 in Fig. 1. Over this surface I5 is deposited a smooth slick-finished protecting film I6.

The term resilient has been herein used to designate that property of the highly stretched racket string to exert a contractive force far in excess of that possessed by the rubber core alone. It is this property which produces a live racket and keeps the strings tight over a long period of use. It is believed this resilient property of the string is caused primarily by the lateral pressure which the rubber core exerts upon the inner wall of the cover, to thereby exert an expanding internal. pressure upon the walls of this cover to contract it longitudinally.

The core I0 may be formed of cut rubber, extruded latex or other forms of rubber or rubberlike materials that are suificiently elastic and durable,

In order that a clearer understanding of the construction of the resilient string of the present invention may be had, an example of one satisfactory construction is given as follows:

Each cord II was formed of six ends I2 of silk 16/18 denier 5 by 3; that is five threads 3 ply silk boiled-off in water and stretched. Eight of these cords I I were helically wound with ten wraps per inch tightly about a stretched cord I0 of cut rubber size 32's.

Before the core I0 was covered it was passed through a supply of adhesive consisting of parts rubber (latex+ 50 parts casein and a small amount of vulcanizing ingredients. This adhesive was applied to the core at 30% total solid and the cords I I were then applied tightly about this wet core.

The tennis .string was later stretched for a number of hours while tensioned by a weight of about 40 pounds. It was then wiped with Formalin (40%) and dried under tension. The rubber of the adhesive was then vulcanized by subjecting the stretched string for 1-5 minutes to a temperature of 250 F. It was then given the finished surface I6 by repeatedly coating it with a mixture of gelatin, glycerine and castor oil and each coat after drying.

After the resilient string of the present invention has been constructed as above described, it may be strung in a racket I! in the usual manner as shown in Fig. 4.

The racket string thus produced was found to have a tensile strength of approximately pounds, and to be tough and durable when strung in a racket and to have good live playing properties.

Having thus described our invention, What we claim and desire to protect by Letters Patent 1s:-

1. A resilient racket string strong enough to be strung as tight as a high grade gut string of similar size, comprising a tensioned rubber core, a strong outer cover therefor formed of a number of cords uniformly arranged about the core as an axis and tightly wrapped helically about the core at a sufficiently small angle to the length of the core to hold the core stretched and under circumferential compression when the racket string is polishing tensioned, and an adhesive binder bonding the.

cords to one another and to the core.

2. A resilient racket string strong enough to be strung as tight as a high grade gut string of similar size, comprising atensioned rubber core, a/ strong outer cover therefor formed of a number of cords of silk arranged about the core as an axis and tightly wrapped helically about the core at a sufficiently small angle to the length of the core to hold the core stretched and under circumferential compression when the racket string is tensioned, and an adhesive binder bonding the cords to one another and to the core,

3. A resilient racket string strong enough to be strung as tight as a high grade gut string of similar size, comprising a tensioned rubber core, a

strong outer cover therefor formed of a number of cords arranged about the core as an axis and tightly wrapped helically about the core at a sufficiently small angle to the length of the core to hold the core stretched and under circumferential compression when the racket string is ten.-

sioned, and an adhesive binder containing latex vulcanized in situ to bond the cords to one another and to the core.

4. A resilient racket string strong enough to be strung as tight as a high grade gut string of similar size, comprising a tensioned rubber core, a strong outer cover therefor formed of a number of cords arranged about the core as an axis and tightly wrapped helically about the core at a sufficiently small angle to the length of the core to hold the core stretched and under circumferential compression when the racket string is tensioned so that the rubber core exerts a strong inflating pressure upon the inner wall of the cord cover and causes the racket string to exert a contractive force far in excess of the strength of the rubber core, and an adhesive binder bonding the cords to one another and to the core. 

